bump phpunit from 9.3 to 9.6

Author: guillaume-sainthillier

composer.json

@@ -26,7 +26,7 @@
     "ext-libxml": "Needed for XSD validation"
   },
   "require-dev": {
-    "phpunit/phpunit": "^9.3",
+    "phpunit/phpunit": "^9.6",
     "ext-libxml": "*",
     "friendsofphp/php-cs-fixer": "3.16.*",
     "phpstan/phpstan": "^1.10",